This invention relates generally to methods and apparatus for computed tomography cardiac imaging, and more particularly to methods and apparatus for non-uniform temporal recording of cardiac images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Computed tomography images of the heart are useful for a number of diagnostic and surgical purposes. At least one known procedure requires that a collection of cardiac phase images be obtained. However, the process of obtaining such a collection is complicated by the fact that the heart does not beat in a uniform temporal fashion. During a single cardiac cycle, there are some times during which the volume of the heart is changing faster than average, and some times during which the volume changes more slowly than average. Currently, when temporal cardiac scanning is performed on a CT scanner, images corresponding to several phases of a cardiac cycle are captured at evenly spaced intervals. The images that are acquired are evenly spaced in time, resulting in an oversampling of certain phases of the cardiac cycle. Other phases are undersampled. Thus, temporal resolution is impaired. It would therefore be desirable for CT imaging apparatus and methods to optimize a collection of cardiac phase images by avoiding over- and undersampling.
There is therefore provided, in one embodiment, a method for imaging a heart of a patient utilizing a CT imaging system including steps of assigning a scanning priority to phases of a representative cardiac cycle of the patient""s heart, selecting phases of the cardiac cycle for scanning in accordance with the assigned scanning priority, and obtaining image slices of the patient""s heart corresponding the selected phases of the cardiac cycle.
The above described embodiment results in a non-uniform temporal scan that provides improved temporal resolution. Moreover, both undersampling and oversampling of phases is avoided by the assignment of priorities, resulting in a more optimized collection of cardiac images.